1. Field of the Invention
The present invention relates generally to the correction of convergence errors created in a CRT employing magnetic deflection of the electron beam, and more particularly to the correction of positional and misconvergence errors in an in-line gun assembly with a self-converging deflecting yoke structure. The invention is particularly adapted to systems which must display information in raster format and/or stroke written format, the latter particularly being degraded if convergence control such as provided by the present invention is not employed.
2. Description of the Prior Art
In the color CRT art, three separate beams are generated by discrete electron guns and focused on a shadow mask disposed on the interior surface of the CRT viewing screen. The beam triad passes through a plurality of pin holes in the mask and then diverges to energize adjacent dot triads of red, green and blue phosphors to provide corresponding color emissions from the face of the CRT. Convergence is said to occur when each of the three electron beams lands on its respective colored phosphor dot of a given phosphor triad for each triad over the entire face of the viewing screen.
For the commonly used delta type electron gun, the three guns are arranged in a triangular configuration and inclined to converge at a central point in the viewing screen. A magnetic deflection yoke is configured to provide a uniform deflection field. Misconvergence is accordingly corrected with extra circuitry, such as disclosed by Carl L. Chase, et al, in U.S. Pat. No. 4,385,259, issued May 24, 1983, Dynamic Convergence Control Apparatus for Shadow Mask CRT Displays, assigned to the assignee of the present invention, and which applies a convergence correction current to the deflection yoke dependent on the beam position. The most critical aspect of deflection yoke design for the delta configuration is a disposal of the windings to establish a uniform magnetic field throughout the region occupied by the scanning beam. The deflection yoke generally consists of two pairs of coils wound at right angles to each other and oriented along the beam axis. Two opposing coils are series connected so that their magnetic fields add. The magnetic lines of flux are oriented horizontally to cause vertical deflection, since the resulting deflection of the electron beam is at right angles to the magnetic lines of flux. A horizontal deflection is produced by the opposing pair of coils whose magnetic lines of flux pass vertically through the neck of the tube.
A simpler approach is that of arranging the electron guns in a linear array to provide substantially converging electron beams at the center of the viewing screen and shaping the deflection coil to provide inherent convergence control by providing a non-uniform deflection field. This technique requires critical alignment of the deflection yoke on the neck of the CRT, along with permanent magnets which may be statically trimmed for optimum convergence correction at the center of the face of the CRT. The misconvergence in one plane can be reduced substantially to zero by deliberately introducing an astigmatic deflection of the magnetic fields into the deflection yoke design. For example, for a vertical gun array, the horizontal field is arranged to provide a barrel shaped deflection and the vertical field to provide a pin cushion deflection. When the resultant magnetic fields are combined, the three electron guns can be substantially converged to a single line at any point at the face of the CRT screen. Thus, convergence correction need be addressed only to a single plane.
To realize the required astigmatic magnetic field distribution on a production basis, the yoke must be wound with a high degree of precision. In one embodiment, each turn of wire is precisely placed into the winding grooves of molded plastic rings that are cemented to each end of a toroidal core. Optimum convergence is then achieved by positioning the yoke in relation to the picture tube neck to properly align the three beams with the center of deflection of the yoke.
A further improvement in the art utilizes a deflection yoke coupled to a convergence coil which is adjustable to minimize misconvergence. However, it has been found that significant positional and misconvergence errors remain uncorrected, particularly with respect to stroke writing displays.
The present invention overcomes the shortcomings of the prior art by an improved CRT convergence circuit design which yields substantial production interchangeability without the need for dynamically modifying the deflection amplifier waveforms, by providing individual adjustment of the convergence network inductance and resistance to match the L/R time constants.